Federal Regulations limit an 18 wheeler driver to 11 hours of driving per day and 77 hours per week. However, surveys have found that many 18 wheeler drivers violate these regulations.
McCartt, A.T.; Hellinga L.A.; and Solomon, M.G. 2008. Work schedules of long-distance truck drivers before and after 2004 hours-of-service rule change. Traffic Injury Prevention 9:201-10.
So when you are travelling on the highway, you should not assume that the truck driver in the 18 wheeler or other large truck is rested and alert. 11 hours of driving is a long time in a good luxury vehicle. Imagine driving an 18 wheeler that long.